It is known that an operation frequency of a circuit unit or an electric power supply voltage supplied to the circuit unit is reduced in a standby state in order to reduce electric power consumption of a semiconductor device.
There is an electric power supply device that utilizes a digital voltage adjustment circuit to reduce the electric power supply voltage supplied to the circuit unit, to a target value. The digital voltage adjustment circuit includes a plurality of switches (which are configured with transistors) that are connected in parallel between an electric power supply line to which the electric power supply voltage is applied and the circuit unit. The voltage adjustment circuit compares the target value and the electric power supply voltage supplied to the circuit unit in a predetermined cycle, and increases the number of switches turned off, when the electric power supply voltage is higher than the target value, in order to reduce the electric power supply voltage.
See, for example, Japanese Laid-open Patent Publication Nos. 2014-57218, 2015-97460, 2004-260933, and 10-215569.
However, it is possible that the conventional electric power supply device causes undershoot in which the electric power supply voltage becomes lower than the target value, due to the adjustment of the electric power supply voltage which is performed when the circuit unit changes from a usual state to a standby state. It is possible that, if the circuit unit includes a memory that uses a predetermined voltage to retain its stored data, the stored data is lost due to the undershoot, for example.
Hence, it is desirable to prevent occurrence of the undershoot at the time of electric power supply voltage adjustment.